1. Field of the Invention
The present invention relates to an organic light emitting diode display and a driving method thereof, and more particularly to an organic light emitting diode display that is adapted to compensate a threshold voltage of a driving thin film transistor to improve a display quality.
2. Description of the Related Art
Recently, various flat panel displays have been developed having reduced weight and bulk, which eliminates the disadvantages of cathode ray tubes. Such flat panel display devices include liquid crystal displays (LCD), field emission displays (FED), plasma display panels (PDP), and electro-luminescence devices (EL), etc.
The PDP has an advantage of having a thin profile and light weight, and is suitable for making large screens because of its simple structure and a simple manufacturing process. However, the PDP has a disadvantage of low luminous efficiency, low brightness levels, and high power consumption. Furthermore, since an active matrix LCD having thin film transistors (TFT) is formed by a semiconductor process, it is difficult to manufacture a large size screen. The active matrix LCD has a disadvantage of high power consumption, as it uses a backlight unit as a light source.
The EL device is classified into an inorganic light emitting diode display and an organic light emitting diode display, depending upon a material of the light emitting layer. The EL device is a self-luminous device. The EL device has an advantage of fast response time, high luminous efficiency, high brightness levels, and a wide viewing angle. The inorganic light emitting diode display has high power consumption and cannot provide the high brightness levels compared to the organic light emitting diode display, and cannot emit a variety of colors using an R color, a G color, and a B color. On the other hand, the organic light emitting diode display is driven at low DC voltage levels, has a fast response time, and provides high brightness levels. As a result, the organic light emitting diode display can emit a variety of colors using an R color, a G color, and a B color, and is well-suited for the next generation of flat panel displays.
Referring to FIG. 1, if a voltage is applied between a first electrode 100 and a second electrode 70 of the organic light emitting diode display, an electron generated from the second electrode 70 moves toward an organic light emitting layer 78c via an electron injection layer 78a and an electron transport layer 78b. Further, a hole generated from the first electrode 100 moves forward in the light emitting layer 78c via a hole injection layer 78e and a hole transport layer 78d. The electron supplied from the electron transport layer 78b and the hole supplied from the hole transport layer 78d collide with each other in the light emitting layer 78c and recombine to generate light. The light is then emitted to the exterior via the first electrode 100 so as to display an image.
FIG. 2 is a block diagram schematically showing an organic light emitting diode display of the related art. Referring to FIG. 2, the organic light emitting diode display of the related art includes an OLED panel 20, a gate driving circuit 22, a data driving circuit 24, a gamma voltage generator 26, and a timing controller 27. The OLED panel 20 has pixels 28 arranged at the intersection of the gate lines GL and data lines DL. The gate driving circuit 22 drives the gate lines GL of the OLED panel 20. The data driving circuit 24 drives the data lines DL of the OLED panel 20. The gamma voltage generator 26 supplies a plurality of gamma voltages to the data driving circuit 24. The timing controller 27 controls the data driving circuit 24 and the gate driving circuit 22.
The pixels 28 are arranged in a matrix on the OLED panel 20. A supply pad 10 and a ground pad 12 are formed on the OLED panel 20. The supply pad 10 receives a high-level voltage supplied from the external high-level voltage source VDD. The ground pad 12 receives a ground voltage supplied from the external ground voltage source GND. For example, the high-level power voltage source VDD and the ground voltage source GND may be supplied from a power supply. The high-level voltage supplied to the supply pad 10 is supplied to each of the pixels 28. Also, the ground voltage supplied to the ground pad 12 is supplied to each of the pixels 28.
The gate driving circuit 22 supplies a gate signal to the gate lines GL to sequentially drive the gate lines GL. The gamma voltage generator 26 supplies a gamma voltage having a variety of voltage values to the data driving circuit 24.
The data driving circuit 24 converts a digital data signal, which is inputted from the timing controller 27, into an analog data signal using a gamma voltage from the gamma voltage generator 26. Furthermore, the data driving circuit 24 supplies an analog data signal to the data lines DL when a gate signal is supplied.
The timing controller 27 generates a data control signal that controls the data driving circuit 24 and a gate control signal that controls the gate driving circuit 22 using a plurality of synchronization signals. A data control signal, which is generated from the timing controller 27, is supplied to the data driving circuit 24 to control the data driving circuit 24. A gate control signal, which is generated from the timing controller 27, is supplied to the gate driving circuit 22 to control the gate driving circuit 22. The timing controller 27 supplies a digital data signal, which is supplied from a scaler (not shown), to the data driving circuit 24.
Each of the pixels 28 receives a data signal from the data line DL to generate light corresponding to the data signal when a gate signal is supplied to the gate line GL. To this end, each of the pixels 28 includes a light emitting cell OEL and a cell driving circuit 30, as shown in FIG. 3. The light emitting cell OEL has a cathode, which is connected to a ground voltage source GND, that is, a voltage which is supplied from the ground pad 12. The cell driving circuit 30 is connected to the data line DL and a high-level voltage source VDD (a voltage which is supplied from the supply pad 10), and is connected to an anode of the light emitting cell OEL to drive the light emitting cell OEL.
The cell driving circuit 30 includes a switching TFT T1, a driving TFT T2, and a capacitor C. The switching TFT T1 has a gate terminal which is connected to the gate line GL, a source terminal which is connected to the data line DL, and a drain terminal which is connected to a node N. The driving TFT T2 has a gate terminal which is connected to a node N, a source terminal which is connected to a high-level voltage source VDD, and a drain terminal which is connected to a light emitting cell OEL. The storage capacitor C is connected between a high-level voltage source VDD and the node N.
If a gate signal is supplied to the gate line GL, the switching TFT T1 is turned-on to supply a data signal from the data line DL to the node N. The data signal supplied to the node N charges the storage capacitor C and is supplied to a gate terminal of the driving TFT T2. The driving TFT T2 controls an amount of current I, which is supplied from a high-level voltage source VDD to the light emitting cell OEL in response to a data signal supplied to its gate terminal, to control an amount of light emitted from the light emitting cell OEL. Furthermore, although the switching TFT T1 is turned-off, a data signal is discharged from the storage capacitor C. As a result, the driving TFT T2 can supply a current I from the high-level voltage source VDD to the light emitting cell OEL to allow a light emitting cell OEL to emit light until a data signal of a next frame is supplied. Herein, the cell driving circuit 30 may be set in a variety of structures other than the above-mentioned structure.
However, in the organic light emitting diode display apparatus which is driven in this manner, if a gate voltage having the same polarity is applied for a long time, a threshold voltage Vth of the driving TFT T2 is raised, thereby changing an operating characteristic of the driving TFT T2. A change of operating characteristics of such a driving TFT T2 is shown by the experimental results in FIG. 4.
FIG. 4 show experimental results where characteristics of a hydrogenated amorphous silicon TFT a-Si:H TFT of a test sample is changed when a positive gate-bias stress is applied to a hydrogenated amorphous silicon TFT for a test sample a-Si:H TFT having a channel width to channel length ratio W/L of about 120 μm/6 μm. The x-axis represents a gate voltage V, and the y-axis represents a current between a source terminal and a drain terminal of a hydrogenated amorphous silicon TFT a-Si:H TFT for a test sample. Each curve represents operating characteristics of a hydrogenated amorphous silicon TFT a-Si:H TFT, where a gate voltage applying time is increased from left to right.
FIG. 4 shows shifting of a threshold voltage of a TFT and an operating characteristics curve according to a voltage applying time when a voltage of about +30V is applied to a gate terminal of a hydrogenated amorphous silicon TFT a-Si:H TFT. If the time that a high voltage of positive polarity is applied to a gate terminal of a hydrogenated amorphous silicon TFT a-Si:H TFT is increased, the operating characteristics curve of the TFT moves to the right, and a threshold voltage of the hydrogenated amorphous silicon TFT a-Si:H TFT is increased (a threshold voltage is increased from Vth1 to Vth4).
As described above, if a threshold voltage of the driving TFT T2 is increased, the TFT T2 becomes unstable. Thus, it is difficult for the organic light emitting diode display to be normally driven. To solve this problem, the organic light emitting diode display of the related art provides a compensation method, which increases a gate voltage of the driving TFT T2 in proportion to the increased threshold voltage to allow an arbitrary current to flow through a source and drain terminals of the driving TFT T2.
However, the organic light emitting diode display of the related art, which provides such a compensation method, continuously increases a gate voltage in proportion to an increase of a threshold voltage of the driving TFT T2, thereby degrading performance of the driving TFT T2. Accordingly, in the organic light emitting diode display of the related art, a threshold voltage of the driving TFT T2 is further increased, so that a degradation of the driving TFT T2 is accelerated. As a result, the display quality of the organic light emitting diode display deteriorates and the life span is decreased.